Tetraceratops

Tetraceratops insignis ("four-horned face emblem") is an extinct synapsid from the Early Permian of Texas. It was originally classified as an early and very primitive member of Therapsida, a group that includes mammals and their close extinct relatives.[1][2] However, it is now seen as a primitive non-therapsid sphenacodont rather than a genuine basal therapsid.[3]

Tetraceratops
Temporal range: Leonardian stage, 275 Ma
↓
Holotype
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Clade: Sphenacodontia
Genus: †Tetraceratops
Matthew, 1908
Type species
†Tetraceratops insignis
Matthew, 1908

Description
Restoration

Tetraceratops is known from a single 90-millimetre-long (3.5 in) skull discovered in Texas in the early 1900s. Contrary to its genus name, Tetraceratops actually has six horns, one pair being on the premaxilla bones, one pair on the prefrontal bones, and one pair on the angular processes of the mandible. When it was discovered and described in 1908, the skull was still embedded in a matrix, and only the premaxilla and prefrontal pairs were visible. In life, thus, it would have resembled a large lizard with four horns on its snout, and a pair of large spines emanating from the corners of its jaw.

In addition to horns, Tetraceratops also had an impressive set of teeth. The second pair of teeth on the maxillary bones were large and fang-like. Likewise the first teeth in the upper jaw were long and dagger-like.

A diagram of the skull

Classification
Hypothetical full-body reconstruction

Tetraceratops was originally identified as a member of a group called Pelycosauria, an evolutionary grade of synapsids more basal than therapsids. It has been variously grouped in the family Sphenacodontidae, which is closely related to Therapsida, and Eothyrididae, which is more distantly related. Recent phylogenetic studies classify it as either a pelycosaur-grade synapsid or the basalmost therapsid, rendering its exact phylogenetic placement unclear.[2][4][5][6] However, a new study by Spindler (in press) concluded that no convincing morphological evidence could be made for a therapsid placement of Tetraceratops and that this genus was better placed as a basal sphenacodontian.[3]

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See also

References
  1. Laurin, M.; Reisz. R. R. (1996). "The osteology and relationships of Tetraceratops insignis, the oldest known therapsid". Journal of Vertebrate Paleontology. 16 (1): 95–102. doi:10.1080/02724634.1996.10011287.
  2. Amson, E.; Laurin M. (2011). "On the affinities of Tetraceratops insignis, an Early Permian synapsid". Acta Palaeontologica Polonica. 56 (2): 301–312. doi:10.4202/app.2010.0063.
  3. Spindler, Frederik (2020). "The skull of Tetraceratops insignis (Synapsida, Sphenacodontia)". Palaeovertebrata. 43 (1): e1. doi:10.18563/pv.43.1.e1.
  4. Sidor, C. A.; Hopson, J. A. (1998). "Ghost lineages and "mammalness": assessing the temporal pattern of character acquisition in the Synapsida". Paleobiology. 24 (2): 254–273. JSTOR 2401242.
  5. Conrad, J.; Sidor, C. A. (2001). "Re-evaluation of Tetraceratops insignis (Synapsida: Sphenacodontia)". Journal of Vertebrate Paleontology. 21: 42A. doi:10.1080/02724634.2001.10010852.
  6. Liu, J.; Rubidge, B; Li, J. (2009). "New basal synapsid supports Laurasian origin for therapsids". Acta Palaeontologica Polonica. 54 (3): 393–400. doi:10.4202/app.2008.0071.


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